As mentioned before, the siren's existence is mainly because of the now somewhat imploding nuclear market. It's not necessarily a flawed siren by any stretch; more so, it's not quite as efficient as the 508 when it comes to effective use of the sound being produced by the chopper.
The Equinox was a special order siren for a while, but FS officially brought it into the 2001 lineup as a lower-pitched alternative to the 130. It's actually less expensive than the 130 from what I've seen, and depending on the kind of environment you're dealing with it would be a better pick as the lower pitch can allow it to outperform a 130 despite the difference in SPL probably. The 2001-130 can pretty much outperform every modern mile range siren and even older ones like the Thunderbolt at 100 feet, but when you put one up in a valley like where I stay the competition's sirens will outrun them, even with a decent SPL difference since the terrain and tree density will eat the high pitch. That's really where lower-pitched sirens start to shine.
With that said, a lot of the reasoning behind why they don't sell well is simply because dealerships don't push them. For the SPL and price it's more advantageous for them to push the 130 since it's more likely to win bids. The 508 cost more and has a lower SPL, and given that the Equinox is 3 dB lower, it makes it hard to sell even with the lower price tag when compared to a Whelen Vortex or a Sentry 16V1T-B or14V-B. There are more 130s going up than 508s and Equinoxes combined, depending on where you are in the country. The 130 dB rating makes them very competitive. I recently discovered a 508 in my area that was installed within the last year. Almost every FS siren that goes up down here is a 2001, so when I saw it on the Alabama map I about choked and had to confirm it in person. Of course my luck the stupid thing didn't test when I tried to record it on Wednesday.
